One of the drawbacks of previously known systems is that they involve coupled movements, namely that having the object located on the other side of the wrist rotating around one axis involves an additional rotation of the object around the other axis, the said drawback being undesirable and one which thus needs to be corrected.
According to a known system represented on FIG. 1, the object A can be guided around two axes R.sub.1 and R.sub.2 by means of two motors M.sub.1 and M.sub.2 introduced into a support S, in fact the forearm of the robot.
The shafts of the two motors M.sub.1 and M.sub.2 are perpendicular to the axis R.sub.2. The shaft of the first motor M.sub.1 ends by a first conical gear wheel 5 engaged with a first toothed conical gear wheel 6, itself engaged with a second toothed conical gear wheel 7; the axis of rotation of the first toothed conical gear wheel 6 is merged with the asis R.sub.2. By means of this train of gears, a dual 90.degree. angle transmission is obtained; the object A is secured to the extremity of a shaft 8 pivoting by rotating the second toothed conical gear wheel 7 and which materializes the axis of rotation R.sub.1.
The shaft of the second motor M.sub.2 ends by a second gear 9, namely a spur gear, and which engages a pinion 10 in rotation around the shaft of the first motor M.sub.1 and linked to a third toothed conical gear wheel 11, itself engaged with a fourth tooth conical gear wheel 12. A 90.degree. angle transmission is thus obtained: the axis of rotation of the fourth toothed conical gear wheel 12, materialized by a shaft 14 turning inside a bearing 15 of the support S and supporting the second conical gear wheel l6, is the axis R.sub.2.
The fourth toothed conical gear wheel 12 and its shaft 14 form an integral part of a fork joint C which in addition includes a clamp 16 secured at one extremity to said fourth wheel 12 and ends at its other extremity by a pivot 17 inside the extension of the shaft 14 and turning (also around the axis R.sub.2) inside a bearing 18 provided inside the support S. The shaft 8 to which the object A is secured is supported by a bearing 13 provided inside the clamp 16.
The rotation of the object A around the axis R.sub.1 occurs by a rotation of the motor M.sub.1 which sets going the transmission constituted by the conical gear wheel 5 and the first and second conical gear wheels 6 and 7. The fork joint C thus remains immobile and, in this instance, the uncoupling of movements effectively exists.
But when the motor M.sub.2 is started, the rotation of the fork joint C--and the object A--around the axis R.sub.2, produced by the setting going of the mechanical transmission composed of the spur gear 9, the toothed wheel 10 and the third and fourth toothed conical gear wheels 11 and 12, is accompanied by a translation of the object A, the shaft 8 and the second toothed conical gear wheel 7 due to the jutting out of the clamp 16. As the first toothed conical gear wheel 6 remains immobile, the second toothed conical gear wheel rotates: rotation around the axis R.sub.2 is accompanied by an undesired rotation around the axis R.sub.1 which is then necessary to correct with the aid of the first motor M.sub.1.
The uncoupling of the rotation movements is thus only partial. This results in a serious complication of the method for operating the robot, this being particularly constricting in the case of production machines on an assembly line where fast repetitive operations are required.
For a more detailed description of a device of this type, reference should be made to the French patent No. 2 504 051.
There are also devices where the motor M.sub.1 is located on the clamp C. The transmission composed of the gears and wheels 5, 6 and 7 is eliminated. This conception makes it possible to uncouple the rotations, but involves a greater heaviness in the fork joint which impairs the speed and facility of its movements. Such devices are described in the European patent No. 0 178 620 and the U.S. Pat. No. 4,073,201.